Image forming apparatus provided with contact-type charger and controller for cleaning charger

Abstract

A charging member is disposed so as to contact a surface of a charge-receiving member which moves relative to the charging member. In conjunction with the movement of the charge receiving member, the charging member removes foreign substance present on the charge-receiving member on the upstream side in the direction of travel of the charge-receiving member at the contact region with the charging member. Simultaneously, a discharge voltage is applied to the charging member so that the charging member charges the charge-receiving member in the area from which said foreign substance has been removed at the contact region on the downstream side of the charge-receiving member in the direction of travel of the charge-receiving member. During a cleaning operation, the charging member retracts from the charge-receiving member and is out of contact therewith in accordance with the number of image formations, black and white ratio, and humidity. The removed foreign substance passes by said charging member in conjunction with the movement of the charge-receiving member.

What is claimed is: 
     
       1. A contact-type charger comprising: a charging member disposed so as to contact the surface of a charge-receiving member, wherein said charge-receiving member moves relative to said charging member, such that the charging member removes foreign substance present on the charge-receiving member on the upstream side in the direction of travel of the charge-receiving member at the contact region with the charging member, and charges said charge-receiving member in the area from which said foreign substance has been removed at the contact region on the downstream side of the charge-receiving member in the direction of travel of the charge-receiving member; and   passing means for making the removed foreign substance pass by said charging member in conjunction with the movement of the charge-receiving member, wherein a discharge voltage is applied between said charging member and the charge-receiving member on the downstream side of an initial contact position in the travel direction, said initial contact position being a location of initial contact between the charging member and the charge receiving member in view of the travel direction.   
     
     
       2. The contact-type charger as claimed in claim 1, wherein the discharge voltage is not applied between said charging member and the charge-receiving member at said initial contact position. 
     
     
       3. The contact-type charger as claimed in claim 1, wherein said charging member includes a discharge electrode which the discharge voltage is applied on and an insulated member between said discharge electrode and said charge-receiving member. 
     
     
       4. The contact-type charger as claimed in claim 3, wherein said discharge electrode has a plurality of tips. 
     
     
       5. The contact-type charger as claimed in claim 1, wherein said passing means includes a mechanism which moves the charging means between a operative position where the charging member is in contact with the charge-receiving member and a non-operative position where the charging member is out of contact with the charge-receiving position.   
     
     
       6. A contact-type charger comprising: a charging member disposed so as to contact the surface of a charge-receiving member, wherein said charge-receiving member moves relative to said charging member, such that the charging member removes foreign substance present on the charge-receiving member on the upstream side in the direction of travel of the charge-receiving member at the contact region with the charging member, and charges said charge-receiving member in the area from which said foreign substance has been removed at the contact region on the downstream side of the charge-receiving member in the direction of travel of the charge-receiving member; and   passing means for making the removed foreign substance pass by said charging member in conjunction with the movement of the charge-receiving member, wherein said charging member includes a flexible electrode which discharge voltage is applied on and an intermediate member which is disposed between said flexible electrode and the charge-receiving member.   
     
     
       7. The contact-type charger as claimed in claim 6, wherein said intermediate member is a semiconductive material. 
     
     
       8. The contact-type charger as claimed in claim 7, wherein said passing means includes a power unit which supplies an electric field to produce a weak and mutually attractive adhesion force between the charging member and the charge-receiving member or a mutually repulsive electric field. 
     
     
       9. The contact-type charger as claimed in claim 8, wherein said power unit applies a voltage to said intermediate member which has equal potential to the potential of the charge receiving member. 
     
     
       10. The contact-type charger as claimed in claim 8, wherein said power unit applies an alternative current voltage to said intermediate member so that the charging member oscillates to allow foreign substance to pass. 
     
     
       11. The contact-type charger as claimed in claim 6, wherein said intermediate member is an electret material. 
     
     
       12. A image forming apparatus comprising: a charge-receiving member on which images is formed;   a charger including,   a charging member disposed so as to contact the surface of said charge-receiving member, wherein said charge-receiving member moves relative to said charging member, such that the charging member removes foreign substance present on the charge-receiving member on the upstream side in the direction of travel of the charge-receiving member at the contact region with the charging member, and charges said charge-receiving member in the area from which said foreign substance has been removed at the contact region on the downstream side of the charge-receiving member in the direction of travel of the charge-receiving member, and   passing means for making the removed foreign substance pass by said charging member in conjunction with the movement of the charge-receiving member; and   a controller which controls an operation of said passing means, wherein   said controller energizes said passing means every predetermined number of image forming operations to make the removed foreign substance pass by said charging member, and   said controller changes said predetermined number in accordance with black to white ratio of images formed on the charge-receiving member.   
     
     
       13. The image forming apparatus as claimed in claim 12, wherein said controller energizes said passing means during a non-image forming period to make the removed foreign substance pass by said charging member. 
     
     
       14. The image forming apparatus as claimed in claim 12, further comprising, a cleaner disposed on downstream side of the charger with respect to the travel direction, said cleaner removing the foreign substance which passes the charging member from the surface of the charge-receiving member.   
     
     
       15. The image forming apparatus as claimed in claim 14, wherein said cleaner includes a developing roller which develops the images on the charge-receiving member with developer.   
     
     
       16. The image forming apparatus as claimed in claim 14, wherein said cleaner includes a transfer roller which transfers the images from the charge-receiving member to a sheet.   
     
     
       17. An image forming apparatus comprising: a charge-receiving member on which images is formed;   a charger including,   a charging member disposed so as to contact the surface of said charge-receiving member, wherein said charge-receiving member moves relative to said charging member, such that the charging member removes foreign substance present on the charge-receiving member on the upstream side in the direction of travel of the charge-receiving member at the contact region with the charging member, and charges said charge-receiving member in the area from which said foreign substance has been removed at the contact region on the downstream side of the charge-receiving member in the direction of travel of the charge-receiving member, and   passing means for making the removed foreign substance pass by said charging member in conjunction with the movement of the charge-receiving member; and   a controller which controls an operation of said passing means, wherein   said controller energizes said passing means every predetermined number of image forming operations to make the removed foreign substance pass by said charging member, and   said controller changes said predetermined number in accordance with ambient humidity.   
     
     
       18. The image forming apparatus as claimed in claim 17, further comprising, a cleaner disposed on downstream side of the charger with respect to the travel direction, said cleaner removing the foreign substance which passes the charging member from the surface of the charge-receiving member.   
     
     
       19. The image forming apparatus as claimed in claim 18, wherein said cleaner includes a developing roller which develops the images on the charge-receiving member with developer.   
     
     
       20. The image forming apparatus as claimed in claim 18, wherein said cleaner includes a transfer roller which transfers the images on the charge-receiving member to a sheet.